Pneumatic tires are formed from a number of different components, including treads and sidewalls. These components impart their respective properties to the pneumatic tires. Treads which make contact with road surfaces, among other components, need to have grip performance, abrasion resistance, tensile properties, and other properties.
Rubber compositions for use in treads of pneumatic tires are usually prepared by adding fillers such as silica or carbon black, and softeners such as low temperature plasticizers, process oils, liquid resins, or resins having a softening point of 160° C. or lower to a rubber component including a diene rubber. Further, thereto are added sulfur and zinc oxide, and optionally vulcanization accelerators such as thiazole vulcanization accelerators (e.g. TBBS, CBS), thiuram vulcanization accelerators (e.g. TBZTD, ZTC), or guanidine vulcanization accelerators (e.g. DPG), and they are kneaded and formed, followed by crosslinking by heat pressing in a vulcanizer to produce pneumatic tires.
For example, Patent Literature 1 discloses a technique that involves incorporation of an alkoxysilane polysulfide, zinc dithiophosphate, and a guanidine derivative with a diene elastomer to promote a coupling reaction, thereby improving abrasion resistance and tensile properties.
It has been known that the use of a diene rubber in the rubber component provides good durability while ensuring good handling stability, fuel economy, and elongation at break, but unfortunately leads to the occurrence of blowing (porosity) during high-temperature running. In particular, in the case of compositions which incorporate at least a certain amount of carbon black or softeners with a diene rubber in order to achieve high grip performance or abrasion resistance, blowing easily occurs because the compositions show high heat build-up and are thus susceptible to breakage of crosslinks and cannot withstand the inflation pressure of the volatile components in the rubber. For racing tires, since in summer the temperature of the tire tread rises up to 100° C. and the internal temperature of the tread rises up to 120° C., no occurrence of blowing is desired even when the tires are run approximately 40 laps of a circuit at such high temperatures. If blowing occurs in the tread during a race, rubber stiffness decreases so that running lap time deteriorates, and further the tread portion may even be broken, chipped, or separated. However, there has not been established a technique that prevents blowing during dry running by the use of a rubber component including a diene rubber.
Meanwhile, the addition of aluminum hydroxide is proposed as a method for imparting wet grip performance to a tread. However, this method is disadvantageous in that it deteriorates abrasion resistance or abrasion appearance (i.e. grainy peaks like waves) after abrasion, and is therefore rarely applied to tires for general public roads.
The deterioration of abrasion appearance after abrasion, specifically, the formation of taller wave-like grainy peaks (ripples), suggests that the rubber has been subjected to excessive tearing or tension during running. In this case, a decrease in abrasion index is usually observed.